Flaws occur in many structures and systems, such as in aircraft, ships, trucks, space vehicles, buildings, bridges, pipes, and tanks. The term "flaw" as used herein refers to the presence of a region of abnormal (or different from what is to be expected) density, composition, or shape within a system and includes but is not limited to the following kinds of conditions: stress cracks, corrosion, pitting and surface wear (i.e., the absence of material at a location), scratches and dents, bent or misshapen members, swelling (whether caused by stress or absorption of fluids), delamination (the local separation of layers by air, moisture, or other material), presence of an extraneous object or material within a system (e.g., an object inadvertently left within a structure or a piece of material entwined in a rivet or bolt), or any deviation from the intended or specified configuration or material composition internal to a system or structure. Usually, a flaw is a local condition such as a crack, a small region of corrosion, or a dent.
It is often desirable to detect whether a structure or system contains flaws, since flaws may eventually lead to system failure. However, inspection for flaws can be difficult, time-consuming, and expensive, especially if these flaws are hidden within the structure or system. It is sometimes possible to use ultrasonic, eddy current, radiographic, tomographic, or other means to detect flaws in test objects, particularly if the flaws are near the surface. However, the accuracy and reliability of these techniques or difficulties with their use limit their acceptance as non-destructive techniques for hidden flaw detection. For instance, ultrasonic measurements are non-local in the sense that the sound waves propagate throughout the system and thus are subject to system interferences. Tomographic and radiographic techniques are typically performed in transmission mode which requires access to multiple sides of the test object. Although backscatter tomography and radiography can be applied, these carry safety and maneuverability implications due to high-intensity radiation sources. In addition, all imaging techniques known in the art (ultrasonic, radiographic, tomographic, etc.) require interpretation of images, which is subjective and qualitative.
U.S. Pat. No. 4,870,669 of Anghaie and Diaz, entitled Gamma Ray Flaw Detection System discloses a gamma-ray flaw detection system and U.S. Pat. No. 5,267,296 of Albert, entitled Method and Apparatus for Digital Control of Scanning X-ray Imaging Systems discloses an X-ray digital imaging system. The '669 patent of Anghaie and Diaz discloses an apparatus that generates a collimated monoenergetic G-ray beam used to examine a test object and infer the presence, location, and size of flaws by processing differential scatter gamma spectra. The method disclosed by the '669 patent, however, simply forms "differential spectra" i.e., the features of the difference between two spectra are used to infer the presence, size, and location of flaws. Subtracting one spectrum from the another forms a differential spectrum. If the two spectra are the same, then the shape of the differential spectrum would be approximately a horizontal line at zero height (with some statistical scatter above and below the value zero). If the spectra differ, the differential spectrum would be a set of discrete data points (or a histogram), some portion of which would be nonzero. The '669 patent further discloses a method of finding the approximate location of the flaw, by making a geometrical inference based on the incident beam, the location of the detector, and the scattering angle. The Anghaie and Diaz method requires that the energy spectrum of the scattered .gamma.-rays be measured. Each data point in an energy spectrum is subject to much larger relative statistical uncertainty than is the total detector response irrespective of .gamma.-ray energy. Thus, one shortcoming of the '669 patent is that the difference of two spectra, each of which is subject to statistical counting uncertainties, is itself subject to relatively larger counting uncertainties. The '669 patent does not disclose a method that accounts for differences that are discernible from these counting uncertainties, making interpretation of results somewhat subjective and uncertain.
The '269 patent of Albert discloses a method relating to the non-destructive detection of flaws hidden in a substrate such as an airplane propeller blade. In particular, the method disclosed in the '269 patent produces images on a display screen that are obtained by digital imaging techniques in a transmission mode. Whereas this has apparent appeal, because an image is formed which can be viewed and on which flaws can sometimes be clearly seen, it suffers from certain significant drawbacks. First, the test sample must be placed between the radiation source and the detectors, which limits practical use of the technique to relatively small samples or requires insertion of detectors inside a system (such as inside an airplane wing). Further, the images must be interpreted in some fashion and the technique can be ambiguous when flaws are not clearly discernible visually. This leads to a need for a trained specialist or to sophisticated pattern-recognition algorithms to interpret the image.
With the foregoing in mind, it is the general object of the invention to provide a method and apparatus adapted to use backscatter of a collimated beam of X- or .gamma.-rays to detect--in a non-destructive fashion--flaws in a substrate that are hidden from direct observation by one or more intervening layers of material.
It is a further object of the invention to provide an apparatus and method, when access to two sides of a small or thin sample is easy to achieve, adapted to transmit X- or .gamma. rays to detect hidden flaws.
It is also an object of the invention to provide an apparatus and non-destructive method of detecting faults that leads to a number, i.e., a figure-of-merit, whose value can be used to indicate the probability of detection of a flaw in a test sample.
It is yet another object of the invention to provide a specific probabilistic method and apparatus adapted to form a figure-of-merit indicative of the probability of a flaw in a test sample whose value incorporates natural statistical uncertainties.
It is another object of the invention to provide a non-destructive method and apparatus for detecting the presence and location of flaw(s) in a test sample having only one side of the sample available for testing.
Another object is to provide a flaw detection method and apparatus, which probes the test sample locally and is thus relatively insensitive to system-wide interferences, and which eliminates the need for formation and interpretation of a visual image of a flaw.
The above and still other objects, features and advantages of the invention will become more apparent upon consideration of the following detailed description of illustrative examples thereof, taken in conjunction with the accompanying drawings.